Question

question 7 of 10
what do directional selection and disruptive selection have in common?
a. they both happen only in small populations.
b. they both decrease genetic variation.
c. they both cause mutations.
d. they both only affect polygenic traits.

Explanation:

• Directional selection: Favors one extreme phenotype. Over time, alleles for the less - favored phenotypes are reduced. For example, in a population of bacteria, if a new antibiotic is introduced, bacteria with a mutation that confers resistance (one extreme) are selected for. Non - resistant bacteria (other phenotypes) die off, reducing genetic variation.
• Disruptive selection: Favors two or more extreme phenotypes over the intermediate. However, as the intermediate phenotype is selected against, the overall genetic variation within the population can decrease in terms of the range of phenotypes that were originally present. For instance, in a bird population where birds with very long beaks can eat one type of seed and birds with very short beaks can eat another type, but intermediate - beaked birds cannot efficiently eat either. If the environment only has these two types of seeds available, the intermediate - beaked birds are selected against. Although there are two extreme phenotypes, the overall genetic variation (in terms of beak length phenotypes) has changed from a continuous range to two distinct groups, and alleles for the intermediate beak lengths are reduced.
• Option A: Both can occur in large populations. For example, in a large population of moths (directional selection due to industrial pollution) or a large population of fish (disruptive selection based on different feeding niches).
• Option C: Selection (directional or disruptive) acts on existing genetic variation (from mutations, but selection itself does not cause mutations). Mutations are random events.
• Option D: They can affect both polygenic (traits controlled by multiple genes, like human height) and single - gene traits. For example, in some insect populations, wing color (a single - gene trait in some cases) can be subject to directional or disruptive selection.

Answer:

B. They both decrease genetic variation.